DESCRIPTION:A basic paradigm in radiobiology is that following cellular exposure to ionizing radiation it is the deposition of energy in the cell nucleus that is responsible for its deleterious biological effects, and that DNA is the principal target for radiation damage. Two rapidly expanding fields of research, radiation-induced genomic instability and bystander effects (BSEs) have caused us to reevaluate this central dogma. In this application we will investigate the role of BSEs, i.e., those effects occurring in cells that were not irradiated, but were bystanders at the time of irradiation, in contributing to the deleterious effects of radiation exposure. We propose that some bystander effect factor (BSEF) is stimulated by cellular irradiation and secreted into the culture medium. We hypothesize that this BSEF is a protein to protein complex. The BSEF could itself be responsible for the observed BSEs in non-irradiated cells and/or could generate reactive oxygen species (ROS) and act in conjunction with these ROS to elicit BSEs. In turn these ROS could stimulate BSEF creating cycles of BSEF/ROS/BSEF/ROS, thus perpetuating a source of damage over time to account for the observed detrimental effects of X-irradiation. These hypotheses will be best addressed in 3 specific aims: Specific aim 1 will test the hypothesis that the SBEF can mimic the deleterious effects of X-ray exposure in non-irradiated cells. Specific aim 2 will test the hypothesis that our unique collection of chromosomally unstable cloned can produce "instability" factor(s) that can induce cytogenetic damage and genomic instability in non-irradiated cells. Specific aim 3 is to isolate, purify, and identify the factor(s) responsible for the BSE, and those "instability" factors produced by unstable cells. We are uniquely qualified to do this. We have in hand the assays for potential BSEs, radiation-induced genomic instability, and a collection of well-characterized chromosomally unstable clones of cells. We have experience in isolating, purifying, and identifying potential BSEF(s) and have the methodology to test candidates. Identifying the BSEF would enhance our understanding of how radiation exerts its deleterious effects and could directly impact risk estimation for radiation exposure.